Proteolipid protein (PLP) is the major protein component of CNS myelin and appears to be a major target of immune responses both in murine models of experimental autoimmune encephalomyelitis (EAE) and human multiple sclerosis. A relapsing-remitting form of EAE (R-EAE) is induced in SJL/J mice following either active immunization with the major encephalitogenic determinant of PLP (PLP139-151) or by the adoptive transfer of peptide-specific T cells. Interestingly, the expression of clinical relapses in PLP139-151-induced R-EAE are, in part, due to the induction of T cell responses against endogenous myelin epitopes (e.g., PLP178-191 distinct from that used to initiate disease (epitope spreading). The relapsing nature of this disease model makes it an ideal system to study the effects of modifying costimulatory signals in animals with a pre-existing autoimmune disease. Activation of encephalitogenic CD4+ T cells required two signals - TcR occupancy and non-cognate interactions between the CD28 molecule on T cells and the B7 family of molecules on antigen presenting cells (APCs). Treatment of mice with activating anti-B7-1 or anti-CD28 mAbs following the initial clinical episode results in a significant exacerbation of the onset, frequency and severity of the subsequent relapses. However, similar treatment with the F(ab) fragments of anti-B7-1 mAb blocked epitope spreading, and significantly ameliorated CNS histopathology and prevented clinical relapses. The aims of this proposal are to further characterize the effects of various monoclonal antibodies and CD28/B7 antagonists administered at varying times during the course of disease. In addition, we will explore the targets and mechanisms of disease exacerbation induced by treatment with intact anti-B7-1 and of disease suppression induced by treatment with the F(ab) fragments. The phenotype and functional responses APCs and of peripheral and CNS-infiltrating T cells specific for PLP178-191, and other myelin epitopes from treated mice will be examined both in vivo and in vitro. An in vitro approach will employ an I-As-expressing 'costimulatory-null' tumor line transfected with B7-1 and B7-2, either alone or in combination, to directly assess the role of these molecules in activation of peripheral and CNS T cells from control and treated mice and in activation of PLP139-151-specific Th0, Th1 and Th2 clones. Induction of cell surface activation and homing antigens, proliferation, patterns of cytokine mRNA and protein expression, and the ability to activate in vivo primed cells and Th1 clones for adoptive transfer of R-EAE will be assessed. Lastly, adoptive transfer experiments and a genetic approach, using SJL/J mice in which CD28, CTLA- 4, B7-1 or B7-2 have been genetically disrupted, will be used to understand the role of each ligand in regulating the induction and expression of R-EAE. These studies should enhance our understanding of the role of costimulatory molecules in regulating epitope spreading in chronic autoimmunity and provide vital information relative to the potential targeting of costimulatory molecules for treatment of pre- existing immune-mediated disorders.